You can do that with Firefox: https://addons.mozilla.org/en-US/firefox/addon/custom-tab-width/ https://addons.mozilla.org/en-US/firefox/addon/custom-tab-wi...

You can do that with Firefox: https://addons.mozilla.org/en-US/firefox/addon/custom-tab-width/ https://addons.mozilla.org/en-US/firefox/addon/custom-tab-wi... (It used to be an about:config setting, but apparently it's now handled by UI CSS, so you could use a CSS file instead of the addon if you wanted)